1275 black exhaust fumes after rebuild

[izzi]

I know this will come across a little strange but I shall try to explain the problem best I can.
Acquired a 1275 rather bodged up with gasket goo engine that was leaking and burning oil.
I stripped down and fitted with new gaskets and seals, head skimmed and reground valves etc;

Using the twin su carbs that came with the engine it now chucks black/grey smoke excessively from the exhaust when reving up.
When I first got the engine there was a definite blue oil smelling smoke, that is now gone and replaced with this black carbon smoke.
I think it must be a carb problem yet the carb set up dare I say seems ok. (spark plugs are black in colour)
I have drained and changed with fresh fuel.( I also have received good help from the forum regarding the replacement timing cover etc;)
I believe I am overlooking or done something wrong,as there was no black smoke prior to my messing about with this refit
Advice would be appreciated.
Roy.

[mike.perry]

Have you adjusted the mixture correctly and balanced the carbs? What sort of crankcase ventilation have you fitted? Plugs should be grey not black

[bmcecosse]

First step - check the carb float bowls are sitting vertically.....if they are 'Mini' carbs on a 'Spridget' manifold they will overflow all the time...... Then check the 'choke' is going fully off. Finally - as above - fine tune once the engine is up to working temperature. Plugs should be light beige/fawn colour after a good run.

[izzi]

Have you adjusted the mixture correctly and balanced the carbs? What sort of crankcase ventilation have you fitted? Plugs should be grey not black

Thanks Mike,
I have adjusted the carbs best I can and you can weaken the mixture on each carb adjustment on tick-over until it stalls the engine. The crankcase ventilation is now from the timing cover (thanks to philthehill,s tips).
The plugs are black due to this problem ( I just mentioned it as info ).
Roy

[izzi]

First step - check the carb float bowls are sitting vertically.....if they are 'Mini' carbs on a 'Spridget' manifold they will overflow all the time...... Then check the 'choke' is going fully off. Finally - as above - fine tune once the engine is up to working temperature. Plugs should be light beige/fawn colour after a good run.

Not to sure if the carbs are from a mini as they were on this car when I got hold of it ( the car is a healey sprite which needs a lot of care) Not my dear reliable Molly.
I am trying to reduce a photo size of the carbs on the car to show, will send asap.
Roy

[izzi]

photo[frame][/frame]
That was a problem reducing the picture. What is the best way to straighten the float ?
Roy

[philthehill]

Firstly - you did put the new seals on the valve guides?
Secondly - I cannot see from your photograph where the breather pipe from the timing cover goes?
The breather should either go through the breather valve (Pt No 13H5191) into the manifold (you have the tapping into the manifold but is plugged with a blanking plug which is just behind the choke cable in the photo) or through the 'Y' connector to both carbs. Your carb set up is the second type but the breather inlets into the carbs are open and NO breather pipes connected to them back through the 'Y' connector to the timing chain cover. That could be part or all of your problem!
See http://www.moss-europe.co.uk/Shop/ViewP ... dexID=1418 for details of installation. The 'Y' connector and associated pipes are items No 82 & 83.
Thirdly - the float chambers appear to be correct for the car. There should be a tag attached to each carb which states the type of carb fitted. The tag should read either AUD136 or AUD327 to be correct. You may have tag AUD662T which has the 'floating' needle AAC but all other aspects of the carbs which will be of concern to you are the same.
Check the needles of AUD 136 & AUD327 for correct fitment. They should be AN - standard, GG - lean and H6 - rich.
If you are not sure how to set up the carbs see http://sucarb.co.uk/technical-hs-type-c ... djustments there is plenty of relative info elsewhere on that web site.
I would suggest that you determine that the engine breather system is fitted correctly and when that is done set the carbs as per the link above.
I would also suggest that all will be correct when you have done the above. Please come back for further info if required.

[izzi]

Yes new valve seals were fitted,
I removed the Y unit from the carbs just to take the photo,
The attach tags must have been previously removed.( when I removed the the float camber tops on the carbs one had 2 gaskets fitted which was rather odd )
I was planning to replace the necessary carb kits and needles this week ( mg spares in Swavesey is only a nice 3/4 hour drive ).
SU carburetters sounds very helpful.

Thanks for this advice.
Roy.

[philthehill]

New valve guide seals fitted good!
I wondered whether you had removed the breather pipe but had to ask the question as there was no indication that you had done so.
The extra gasket under the float chamber lid I suspect was an attempt to adjust the fuel level in the float chamber so you will need to check the float levels are correct before attempting to adjusting the fuel/air mixture.
The tags get lost over time but it is of no consequence if they are missing. If you go on the Moss web site/carb parts you can cross reference the engine number to the correct carbs so you get the right overhaul kits.

[izzi]

Will do, and thanks again.
Roy.

[bmcecosse]

As long as the float bowls are vertical - that's all that really matters. If you take the bells off the carbs and look down at the jets, you should see the fuel level 'just' below the bridge. Not quite so handy on the mech pump on the Spridget - but you could turn the engine over a couple of times (plugs out) to get the pump to deliver petrol.

[JOWETTJAVELIN]

Black smoke is undoubtedly a rich mixture as opposed to blue smoke which is oil. Did you renew the piston rings and have the bores honed or rebored? The engine would also hunt on an over-rich mixture.

If I were you I'd get rid of the twin carb setup which is pointless on a standard A-Series and go for one of the later 1970s/80s carbs.

[bmcecosse]

The twins are indeed pointless (and hamper a 1275 engine badly...) -BUT - it's a Spridget and I guess an element of keeping it standard is involved - plus - a big single on a Spridget makes a dent in the bonnet..........

[mike.perry]

Twin carbs might not be as good but they have that classic look[frame][/frame]

[philthehill]

Twin 1 1/4" on a 1275cc 'A' series are not pointless if maintained and tuned properly. The main problem is that people do not fully understand the tuning process and they end up with a badly running engine with a rich or weak mixture and the carbs out of balance. There is plenty of room for further tuning of the carb(s) to pass more air/fuel without going too far from originality and without costing a fortune. According to David Vizard a pair of standard twin 1 1/4" SU cards on an 'A' series have a good power range from 45bhp to 70/75bhp tailing off over 75bhp. He makes a very good argument for using twin 1 1/4" SU carbs in his various publications.
Having used single H2 (1 1/4") & H4 (1 1/2"), twin H2, HS2, H4 (1 1/2"), HS4 and HD4 (1 1/2" with diaphragm main jet) I have great respect for the 1 1/4" SU carb and it has great tuning potential so it should not be knocked and dismissed as pointless.

[bmcecosse]

The Vizard books I've read say quite the opposite! On the A series each shared port sees the carb attached to that port - it doesn't get any benefit from the second carb on the other port......especially on the crappy std BMC inlet manifold! So a large single on a GOOD manifold is the easiest way forward. The 1071 S for example makes hardly any extra power over a 970 S, and similarly the 1275 S makes little extra over the 1071..... They were so fitted because it was 'expected' - and because it limited the power (and so limited warranty claims) as sold to punters over the counter. Fitting larger carbs to the larger engines immediately jumps the power very considerably, but so does fitting a large single ! There is nothing special about setting up twin SUs - a length of rubber tube and job done !

[philthehill]

bmc
Whilst I agree with you in some ways just to dismiss the twin 1 1/4" SU carb set up as pointless is completely absurd! I have the David Vizard book 'How to Modify your Mini' in front of me as I write and he does not dismiss the twin 1 1/4" SU as completely as you do. Yes the air flow is not perfect but neither is it perfect with a larger single SU carb as the five port arrangement of the cylinder head severely keeps the flow from being perfect. The inlet port shape and the 5 port arrangement convinced me to purchase a Manx Racing 940 head modified to give a 7 port arrangement to better the flow.
There is much more to getting a better air/fuel flow than just putting a bigger single carb onto a 1275cc engine.
Yes setting up a twin SU arrangement to you and me might be simple but there are a lot of people out there who are unable to get it right.

[bmcecosse]

The main 'advantage' of the twin carbs when fitted to the 1098 Spridget (with 295 head and a 3 branch manifold and straight through exhaust too!) is the elimination of the 'hot spot' on the manifold -resulting in a dramatic jump from 48 bhp at 5100 to 55 bhp at 5500, and torque leaping from 60 ft lb at 2500 to 62 ft lb at 3250.... The tiny twins don't flow any better than a single - on a GOOD manifold with NO heating. Each stroke of a piston only sees the bore of one carb. The bigger the better! And yes -I was involved with one of the very first 8 port Arden heads in 'works assisted' private owner hands (not mine I hasten to add) on a 1275 S - now that made a HUGE difference on a rally engine (with amazing torque even on a 649 cam) on twin webers, but strangely on a race engine a 5 porter/angle valve head was proven to give slightly better top end power - on split webers - probably because of 'ramming' effect in the shared inlet ports. But the torque was terrible compared to the 8 porter, so not the best for a rally engine.

[philthehill]

bmc
Just to clarify a point to other post readers the standard 649 cam will not fit an Arden 8 port head as the cam lobe sequence is/was different! The rally cam is/was C-AEG636 (marked AEG637) and which had the same profile as the 649; the race cam is/was C-AEG 599 (marked AEG600) which had a super sprint profile.
I used to frequent the Arden Works as it was the local supplier of Weber Carb parts and I always came away drooling over the quality of the 8 port heads which you could see being manufactured/machined behind the counter.

[bmcecosse]

Yes , well obviously I am aware of all that - the ref to 649 was to indicate the profile... In fact - we found the 'super sprint' to be poorer than the 'sprint' cam when tried extensively on the rollers. No top end gain, and much loss lower down compared to the sprint. A step too far, we thought anyway - although perhaps it worked better on a very high revving 999 engine.